Waveform converter for use with an electronic musical instrument and capable of controlling the duty factor of a rectangular wave tone signal

ABSTRACT

A waveform converter for an electronic musical instrument wherein an input rectangular wave tone signal is supplied to a frequency-voltage converter to generate a D.C. voltage control signal having a voltage level corresponding to the frequency of the input rectangular wave tone signal; and a one-shot multivibrator is triggered by the input rectangular wave tone signal to produce an output rectangular wave tone signal having the same frequency as the input rectangular wave tone signal. The one-shot multivibrator comprises duty factor control means responsive to the D.C. control voltage signal for determining the duty factor of the output rectangular wave tone signal.

BACKGROUND OF THE INVENTION

This invention relates to a waveform converter used with an electronicmusical instrument and capable of controlling the duty factor of arectangular wave tone signal.

With a conventional electronic musical instrument, key depression causestone signals to be selectively taken from tone generators. In this case,the tone signals are given desired tone colors by tone-coloring filters.The tone signal generators are formed of a master oscillator and afrequency divider chain consisting of cascade-connected bistablemultivibrators. Tone signals produced by the tone generators have arectangular wave and a duty factor whose value is chosen to be 50%.

When voiced by the same type of tone-coloring filter, rectangular wavetone signals having a 50% duty factor and those having other dutyfactors present different tone colors. If, therefore, the duty factorsof rectangular wave tone signals could be freely varied, then anelectronic musical instrument would provide a more improved performance.

SUMMARY OF THE INVENTION

It is accordingly the object of this invention to provide a waveformconverter capable of controlling the duty factor of an outputrectangular wave tone signal on the basis of the frequency of an inputrectangular wave tone signal.

According to a waveform converter of this invention, a frequency-voltageconverter, when supplied with an input rectangular wave tone signal,produces a D.C. voltage control signal having a voltage levelcorresponding to the frequency of the input rectangular wave tonesignal; and a one-shot multivibrator which is triggered by an inputrectangular wave tone signal to produce an output rectangular wave tonesignal is provided with duty-factor control means which defines the dutyfactor of the output rectangular wave tone signal in response to theD.C. voltage control signal.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of a waveform converter embodying thisinvention;

FIG. 2 is a circuit diagram of a frequency-voltage converter of FIG. 1;

FIGS. 3 and 4 are waveform diagrams useful in explaining the operationof the frequency-voltage converter;

FIG. 5 is a circuit diagram of a one-shot multivibrator of FIG. 1; and

FIG. 6 is a block diagram of an electronic musical instrument using thewaveform converter of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, referential numeral 11 denotes an input terminaladapted to receive an input rectangular wave tone signal 12 having aduty factor of, for example, 50%. The input rectangular wave tone signal12 is supplied to a frequency-voltage converter 13 and a one-shot ormonostable multivibrator 14. The frequency-voltage converter 13 producesan output voltage whose level corresponds to the frequency of the inputrectangular wave tone signal 12. An output voltage from thefrequency-voltage converter 13 is conducted to a variable duty factorcontrol means included in the one-shot multivibrator 14. Therefore, anoutput rectangular wave tone signal generated by the one-shotmultivibrator 14 when triggered by the input rectangular wave tonesignal 12 has a duty factor determined by the output voltage from thefrequency-voltage converter 13. At an output terminal 15 connected tothe one-shot multivibrator 14 appears an output rectangular wave tonesignal 16 having a different duty factor from that of the inputrectangular wave tone signal 12. It will be noted that the inputrectangular wave tone signal 12 and output rectangular wave tone signal16 have the same repetitive frequency.

FIG. 2 is a circuit diagram of a frequency-voltage converter included inthe waveform converter of FIG. 1. A first differentiation circuitcomprised of a capacitor C1 and resistor R1 is connected between theinput terminal 11 and negative power supply terminal 17. The junction ofthe capacitor C1 and resistor R1 is connected to the base of an NPNtransistor Q1 whose collector is connected to ground through a resistorR2 and whose emitter is connected to the negative supply terminal 17. Acapacitor C2 is connected between the collector and emitter oftransistor Q1. The collector of transistor Q1 is connected through afield effect transistor Q2 to one end of a capacitor C3, the other endof which is connected to the negative power supply terminal 17.

A second differentiation circuit comprised of a capacitor C4 andresistor R3 is connected between the input terminal 11 and ground. Thejunction of the capacitor C4 and resistor R3 is connected to the cathodeof a diode D1. The anode of diode D1 is connected to the base of a PNPtransistor Q3 whose emitter is grounded, and whose collector isconnected to a negative power supply terminal 18 through a resistor R4.The collector of transistor Q3 is connected to the gate of the fieldeffect transistor Q2 through a diode D2 and resistor R5. A biasingresistor R6 is connected between the collector of transistor Q1 and thegate of field effect transistor Q2.

An output voltage from the capacitor C3 is delivered to the oututterminal 19 of the frequency-voltage converter 13 through a high inputimpedance buffer circuit comprised of transistors Q4, Q5 and resistorsR7, R8, R9.

There will now be described by reference to FIGS. 3 and 4 the operationof the frequency-voltage converter 13 arranged as described above. Whenan input rectangular wave tone signal having a waveform shown in FIG. 3Ais supplied to the input terminal 11 and thereafter differentiated bythe first differentiation circuit comprised of the resistor R1 andcapacitor C1 and the second differentiation circuit comprised of theresistor R2 and capacitor C2, then negative- and positive-going spikepulses are formed which correspond to the negative- and positive-goingtransitions of an input signal as shown in FIG. 3B. The positive-goingones of the spike pulses produced by the capacitor C1 and resistor R1drive the transistor Q1 from cutoff into saturation causing thecapacitor C2 immediately to be discharged. The transistor Q1 is in acutoff condition during a time interval between the respectivepositive-going spike pulses. As the result, the capacitor C2 is chargedfrom -12 volts toward zero volt as shown in FIG. 3C at a time constantdefined by the values of resistor R2 and capacitor C2.

On the other hand, the transistor Q3 is driven by the negative-goingones (shown in FIG. 3D) of the spike pulses formed by the capacitor C4and resistor R3 with the resultant transition of its collector voltage.The field effect transistor Q2 is enabled in response to theabove-mentioned transition, causing the capacitor C3 to be coupled inparallel with the capacitor C2. At this point, the voltage of thecapacitor C2 is transferred to the capacitor C3. The point of time atwhich the voltage of the capacitor C2 is to be transferred to thecapacitor C3, namely, the point of time at which the field effecttransistor Q2 is enabled depends on the frequency of the inputrectangular wave tone signal 12. Therefore, the voltage level of thecapacitor C2 to be transferred to the capacitor C3 which is indicated byG in FIG. 3C depends on the frequency of the input rectangular wave tonesignal 12. Obviously, therefore, the output terminal 19 of the highimpedance buffer circuit impressed with the voltage of the capacitor C3produces an output voltage whose level is a function of the frequency ofthe input rectangular wave tone signal 12. FIG. 4 indicates a case inwhich the input rectangular wave tone signal 12 has a lower frequencythan in FIG. 3. As seen from FIG. 4, the lower frequency of the inputrectangular wave tone signal 12 causes an output voltage from thefrequency-voltage converter 13 to become larger in the positivedirection with the voltage level of -12 volts taken as the reference.

FIG. 5 is a circuit diagram of a one-shot multivibrator 14 included inthe waveform converter of FIG. 1. An input rectangular wave tone signal12 supplied to the input terminal 11 is differentiated by a resistor R10and capacitor C5. Due to the presence of a diode D3, negative-goingspike pulses generated by the resistor R10 and capacitor C5 trigger aone-shot multivibrator comprising a pair of transistors Q6, Q7 which arecross-coupled to generate output pulses at an output 15 coupled to thecollector of transistor Q7. A capacitor C6, resistor R11 and transistorQ8 jointly constitute duty factor control means of the multivibrator.The base of transistor Q8 is connected to a control terminal 20 coupledto the output terminal 19 of the frequency-voltage converter 13.Consequently, the transistor Q8 acts as a variable resistor elementwhich presents a resistance corresponding to the level of an outputvoltage from the frequency-voltage converter 13 between the collectorand emitter. Thus, it will be seen that the duty factor of an outputrectangular wave tone signal generated at the output terminal 15connected to the collector of the transistor Q7 becomes a function ofresistance between the collector and emitter of the transistor Q8,namely, a function of the level of an output voltage from thefrequency-voltage converter 13. Where a waveform converter comprises afrequency-voltage converter 13 and one-shot multivibrator 14, thecircuit arrangements of which are indicated in FIGS. 2 and 5respectively, then the duty factor of the rectangular wave output ismore increased, as the frequency of the rectangular wave input is morereduced.

FIG. 6 shows an electronic musical instrument using the waveformconverter of this invention. A known latching selector 30 selectivelydraws out from tone generators 32 a tone signal corresponding to thenote of a depressed key on a pedal keyboard 31 and continues to storeand draw out the tone signal thus selected until another key isoperated. The tone signal derived from the latching selector 30 issuccessively subjected to frequency division by 1/2 dividers 33, 34 toprovide 4', 8' and 16' tone signals. These 4', 8' and 16' tone signalsare supplied to the corresponding waveform converters 35a, 35b, 35c ofthis invention to be converted into tone signals whose duty factorscorrespond to the respective frequencies. Output signals from thewaveform converters 35a, 35b, 35c are sent forth to tone-coloringfilters 37 through the known gates 36a, 36b, 36c enabled by the pedalkeyboard 31. Output signals from the tone-coloring filters 37 areconducted to sound reproducing means (not shown).

What is claimed is:
 1. A duty factor controlling waveform converter foruse with an electronic musical instrument comprising:an input terminalfor receiving an input rectangular wave tone signal; an output terminal;a frequency-voltage converter coupled to said input terminal to receivethe input rectangular wave tone signal and produce a control voltagesignal whose level corresponds to the frequency of the input rectangulartone signal; a one-shot multivibrator coupled between said input andoutput terminals to generate at said output terminal an outputrectangular wave tone signal having the same frequency as the inputrectangular wave tone signal, said one-shot multivibrator including acontrol terminal coupled to receive the control voltage signal and meansresponsive to the control voltage signal for determining the duty factorof the output rectangular wave tone signal to be produced by saidone-shot multivibrator in accordance with the voltage level of thecontrol voltage signal.
 2. The waveform converter according to claim 1wherein said frequency-voltage converter comprises a first capacitor anda first resistor coupled in series across a power supply source; firstand second differentiating circuit means coupled to receive the inputrectangular wave tone signal and each adapted to produce first andseccond spike pulses alternately in response to application of the inputrectangular wave tone signal; a second capacitor; a switching meanscoupled in parallel with said first capacitor and responsive toapplication of the first spike pulse from said first differentiatingcircuit means to discharge said first capacitor; gate means coupled tosaid second differentiating circuit means and responsive to occurrenceof the second spike pulse from said second differentiating circuit meansto couple said second capacitor in parallel with said first capacitor.